The present invention relates to a light-sensitive composition, light-sensitive photographic element prepared from this composition, and a process for the preparation of a printing plate from this composition.
The invention more particularly relates to a light-sensitive mixture which contains a substantially water-insoluble binder, which is soluble in aqueous-alkaline solutions, and a light-sensitive compound. The invention further relates to a light-sensitive photographic element composed of a support and a light-sensitive layer comprising the aforesaid composition. The invention still further relates to a process for the preparation of a printing plate by exposing, developing and heating the produced photographic element.
Processes for the preparation of printing plates by heating or baking the image layer of exposed and developed copying materials have been disclosed, for example, in British Patent Specifications 1,151,199 and 1,154,749. In these processes, a light-sensitive copying material, which is either positive-working and preferably contains an o-quinone-diazide or is negative-working and preferably contains a p-quinone-diazide, is exposed imagewise, developed and then heated to a temperature above 180.degree. C. for such a period and to such a temperature level that the image background, previously developed cleanly, is stained by thermal decomposition products of the image layer. Subsequently, the image background is cleaned again by once more treating it with developer solution. Normally, temperatures of 220.degree.-240.degree. C. are used for heating, and the duration of heating is about 5-60 minutes. This post-treatment achieves a hardening of the printing layer and hence a considerable extension of the print run. When the baking temperatures are at the upper limit of the indicated range, the required baking times are of course relatively short. It has been found that the use of relatively high temperatures in this process is disadvantageous. On the one hand, these temperatures cause the formation of a relatively firmly adhering precipitate in the background areas, and this can be cleanly removed only by means of quite aggressive solutions, and there is also a risk of an attack on the desired printing image. Moreover, at the relatively high baking temperatures or with the relatively long baking times, there is a risk of the conventionally used supports of aluminum being deformed and thus of the printing plate obtained being damaged or rendered useless.
As described in German Offenlegungsschrift 2,626,473, the first-mentioned disadvantage can be avoided by coating the background surface, before baking, with a layer of a water-soluble organic substance or an inorganic salt, which layer can be readily washed off after baking. Related treatments may be found in U.S. Pat. No. 4,191,570. To prevent a deformation of the support, it would be desirable however, to be able to bake at lower temperatures.
German Offenlegungsschrift DE-A 30 39 926 discloses a light-sensitive mixture of the type described above, which, in addition to a light-sensitive compound and an alkali-soluble binder, contains a phenol derivative with 2 to 4 hydroxymethyl groups. By the addition of the phenol derivative, the baking temperature can be reduced to less than 200.degree. C. It is, however, a disadvantage of this material that it possesses an unsatisfactory developer resistance and a flat gradation. Also, it has a reduced shelf life which is due to its content of reactive phenol derivative.
The instant invention provides a light-sensitive mixture, and a photographic element prepared from this mixture, which enables baking to be carried out at lower temperatures without substantially reducing the shelf life and developer resistance of such elements.